Calculates the topographic index of a basin defined as: 
<table>
<tr>
<td>
   log (A/s) - mu
</td>
</tr>
</table>
where:
<ul>
<li>A is the contributing area in one point</li>
<li>s the slope</li>
<li>mu = 1/N sum<sub>i</sub> log(A<sub>i</sub>/s<sub>i</sub>) is the mean value of the logarithm over the whole basin (N is the number of pixels belonging to the basin)</li>
</ul>
<br>
It is an index which is necessary to recognize the sites generating
dunnian surface flow in a similar way. Sites with higher topographic
index become saturated before than sites with lower topographic
index.
<br>
<br>
<h3>Inputs</h3>
<ul>      
<li>the file containing the matrix of the contributing areas (obtained with DrainDir or Tca)</li>
<li>the file containing the matrix of the slope (obtained with Gradient or Slope)</li>
</ul>
<br>
<br>
<h3>Output</h3>
<ul>      
<li>the file containing the matrix of the topographic indexes</li> 
</ul>
